10 June 2006

The previous post in this series was about Littoraria irrorata. Unlike the latter, however, Batillaria minima prefers the ocean side of the line between the sea and the land, if there is such a line, and it is active only when it is fully submerged. I photographed and examined these snails during my recent trip to Tampa, Florida.

These snails live in the intertidal zone. Any slow-moving animal that makes the intertidal zone its permanent home needs to have evolved the ability to withstand desiccation and heat, sometimes lasting for hours. I suspect that such traits were crucial preadaptations for the evolution of true terrestriality in gastropods. Batillaria, however, hasn't evolved far enough to step permanently to the "other side". When the tide is out, these snails withdraw into their shells, closing their apertures with a soft operculum.

Note the tiny barnacles to the right of the snail.

One difference between B. minima and L. irrorata is how they respond to disturbance at low tide. If you pick a L. irrorata from where it has attached itself and place it on the sand, the snail will come out and crawl to another vertical post. B. minima, on the other hand, stays sealed in its shell wherever it is placed. This simple test, which I tried, demonstrates that L. irrorata is almost a terrestrial snail, whereas B. minima has a long way to go, if it is headed in that direction.

3 comments:

I've been looking at your previous posts on these types of snails that live on the sea-land margins and it struck me that their shells seem thicker than say the shells of land snails. Is this so, do you think, and is it related to where they ive too?

The shells of most marine gastropods are thicker (relative to overall shell size) than those of terrestrial gastropods. Many factors, including predation pressure, wave exposure (for those who live at rocky shores) & even calcium carbonate dissolution rate, could affect shell thickness.